1. Field of the Invention
The invention is in the field of the conversion of guided modes of light waves in integrated optical components. More particularly, the invention relates to a mode converter based on the principle of periodic coupling between guided modes of a light wave propagating in a channel-type optical waveguide. In addition, the invention relates to an optical input section for a coherent optical receiver in which such a mode converter is used.
2. Prior Art
In a coherent optical receiver such as can be used in a coherent optical network, a laser is usually incorporated as local oscillator. The light from said laser is mixed with an optical signal received from such a network by the receiver. Since the light transmission through the network is generally not polarization-preserving, the polarization of the optical signal received is undefined. The optical signal received is therefore first split into two polarization components TE and TM, which are then processed separately. This is done by mixing with the light of the local oscillator either directly before or directly after splitting. This technique is known by the term `polarization diversity.` This means, however, that the light of the local oscillator must also contain both polarization components in order to have mixing components which correspond in polarization with the two polarization components of the optical signal received. A laser which is standard in this connection and has a wavelength of the emitted light in the near infrared transmits, however, only TE-polarized light. To obtain the other polarization component consideration could be given to tilting the laser through a suitable angle. However, in an integrated design of the coherent optical receiver, in which the laser is co-integrated, tilting the laser is troublesome, if not impracticable. It is therefore first necessary to convert a portion of said TE-polarized light into TM-polarized light with the aid of a polarization converter or rotator. A polarization converter is understood as meaning a device with which a known portion of one polarization component, TE or TM, in the optical signal at the input of said device is converted into the other polarization component TM or TE respectively, at the output, with a well-defined phase with respect to the one polarization component. A polarization rotator is such a type of device in which, however, a phase shift is uncontrolled. Such TE/TM polarization converters and rotators are known per se, for example from references [1], [2] and [3] (see under D.). Reference [1] discloses a polarization converter for optical waves which is able to convert any input polarization into any desired output polarization. This known converter comprises a polarization rotator sandwiched between two phase shifters. Both the phase shifters and the polarization rotator are based on electro-optical modification of the propagation of the TE component and the TM component. The actual conversion of a fraction of the one component into the other component (TE.rarw..fwdarw.TM) with identical intensity takes place in the polarization rotator. In this connection, use is made of a periodic electrode structure provided over a suitably chosen length on top of an optical waveguide in order to bring about a periodic coupling between the two polarization components using suitably chosen, adjustable control voltages. As a consequence of a repeated coupling of this type, it is possible, depending on the chosen control voltage, cycle length and number of couplings, to convert a desired portion of the one component into the other. The polarization converters known from the references [2] and [3] also make use of the principle of periodic coupling between the two polarization components in an optical waveguide on the basis of electro-optical effects with the aid of a periodic electrode structure. Reference 9 discloses a fiber-optical analog of the polarization converter according to reference 2, based on birefringence due to mechanical stress effects. A periodic coupling is achieved in said analog by bringing about a periodic mechanical pressure in the longitudinal direction on a monomodal birefringent fiber or a bimodal fiber using a comb-type pressure device, the transverse pressure exerted by the latter on the fiber being capable of being controlled piezoelectrically. These known converters have the great advantage of electrical controllability, and they are consequently widely applicable, even in the case indicated above. However, they have the drawback that in applications in which a fixed fraction always has to be converted, such a controllability is in fact superfluous, and therefore makes a circuit such as the abovementioned coherent optical receiver unnecessarily complicated and makes the integrability thereof difficult.